Aluminum had previously dominated the materials for making trays in which frozen foods may be both stored and cooked in an oven. The advent of microwave ovens has created a need for non-metallic trays which may be safely heated in both conventional and microwave ovens. Such trays should retain reasonable strength and dimensional stability over a range of temperatures from about -18.degree. C. to about 230.degree. C.; they must resist embrittlement at the lower temperatures, distortion and yellowing at the higher temperatures, and should resist torsion and flexion sufficiently to allow removal of a hot tray and contents from an oven without sagging and spilling the contents. Materials such as paper and various polymers, including polyesters such as poly(ethylene terephthalate), have been used with varying degrees of success.
McTaggart, U.S. Pat. No. 3,960,807, has used modified poly(ethylene terephthalate) to make ovenable trays. This modified poly(ethylene terephthalate) contains a "crack-stopping" agent, which he teaches is a non-conjugated polyolefinic material, and a nucleating agent, which he teaches is a finely divided, inorganic material.
Gartland, U.S. Pat. No. 4,463,121, teaches that the inorganic nucleating agent of McTaggart, above, may be omitted while retaining adequate crystallization times. Gartland further teaches optional use of a heat stabilizer, which he discloses as substituted phenols, bisphenols and substituted bisphenols, aromatic amines, organic phosphites and the like.
The polyester resins are formed into trays by first extruding a sheet of the polyester, then stamping out a tray blank and thermoforming the blank in a tray mold. For this operation to be successful, the extruded sheet should be largely amorphous so that the tray may be readily thermoformed at temperatures up to about 200.degree. C., but the tray composition, after thermoforming, must crystallize to form a rigid structure that will not deform at temperatures well above 200.degree. C. Such a crystallization must be rapid, because requirements for holding the formed tray for any significant period of time at high temperatures, as in the hot tray mold or in a heated chamber, reduce the number of trays that can be produced in a given time. The crystallization must not, however, be too rapid; the composition must allow time for quenching the extruded sheet prior to crystallization; it must also allow time for re-heating to thermoforming temperatures and for the thermoforming step itself.
Accordingly, an object of the present invention is a composition suitable for making ovenable trays that will crystallize rapidly but not instantaneously at thermoforming temperatures. Another object of the invention is an ovenable tray that is useful for storing foods at temperatures as low as -18.degree. C. and for containing foods being cooked at temperatures as high as 230.degree. C. Other objects of the present invention will be apparent from the specification and claims below.